Photovoltaic (PV) panels are often used to recharge batteries, or to provide power to the grid through grid-tie inverters. PV panels often, however, provide less output power than expected from known device efficiency and illumination.
One reason that PV panels may deliver less than optimum power is that their best power output under typical conditions is often at a voltage that is not well matched to battery charging voltages, or other constant-voltage loads. This happens in part because typical panels are temperature sensitive, they must have sufficient cells connected in series to provide battery charging voltage at high temperatures, this cell count becomes excessive at low temperatures where PV cells produce their best output voltage. Similarly, best power output voltage may change with illumination changes. Other losses occur when any one series-connected PV cell in a panel generates less current than other cells in the panel—barring additional circuitry, the output current of a series string of PV cells is effectively limited by photocurrent produced in the weakest, or most shaded, cell.
Since shading affects photocurrent produced in cells, often limiting current production of a series string of cells to that of a most-shaded cell of the string, un-shaded cells in the same series strings may yield substantially less power than they are otherwise capable of. Further, shading of cells may vary with time of day, sun angle, obstruction position, and even the position of wind-blown leaves or other debris on a panel.
Maximum Power Point Tracking (MPPT) devices are frequently connected between an array of PV panels and a load, such as a battery. They typically have a DC-DC converter that converts an input power at a panel voltage to an output power for the load at a load voltage, and control circuitry that seeks to find a panel voltage at which the array of PV panels produces maximum power. The DC-DC converter of the MPPT device serves to decouple the panel and load voltages.
Typically, MPPT devices are not embedded within panels, but are separate devices wired between an output of a panel, or an array of panels, and the load.